Pressure control for flare systems

ABSTRACT

A small positive pressure is maintained on a flare system by injecting a stream of gas at high velocity into the flare line at a location downstream of all other connections to the line and in a direction counter to the direction of flow of gases in the line. The gas injected is a non-reactive gas free of uncombined oxygen, and may typically be nitrogen.

United States Patent 1 [11] 3,726,298

Merritt [451 Apr. 10, 1973 PRESSURE CONTROL FOR FLARE 2,957,306 10/1960 Amnello ..l37/604 X SYSTEMS 3,485,095 12/1969 Hirata 2,692,764 10/1954 Hanson ..l37/604 X [75] Inventor: Arthur D. Meritt, Newark, Del.

73 Assignee: Sun Oil Chemical Company, a y ExaminerAlan Cohan Claymont, Del. Att0rneyGeorge L. Church et al.

[21] Appl' 166515 A small positive pressure is maintained on a flare system by injecting a stream of gas at high velocity [52] US. Cl ..137/13 into the flare line at a location downstream of all other [51] Int. Cl. ..G05d 1/16 nnections to the line and in a direction counter to [58] Field of Search..... ..137/13, 81.5, 604; the direction of flow of gases in the line. The gas in- 138/45 jected is a non-reactive gas free of uncombined 0xygen, and may typically be nitrogen.

[56] References Cited 3 Claims, 2 Drawing Figures UNITED STATIESPA'IENTS 3,086,357 4/1963 Rubin ..137/l5.2

TO OTHER PROCESS UNIT 5 (PLANT s VIA COLLECTING LINES GASES FROM No.2 HYDROGEN P L ANT ETHYLENE PLANT PATENTEnAPRmlsza 3,726,298

TO OTHER PROCESS I umrs '(PLANTS2 FIG. I. VIA COLLECTING LINES CLE E R0 M *Mmm PLANT I f m ETHYLENE- E PLANT I I Y TO MAIN VIA VALVE5 PLANT INVENTOR;

Y ARTH 0. MERITT z fifi ATTY.

PRESSURE CONTROL FOR FLARE SYSTEMS This invention relates to pressure control, and more particularly to a method and apparatus for increasing the pressure in a line or system through which gas is flowing, such as a flare system. A

In present petroleum refinery and petrochemical complexes it is common practice to provide a flare system for safe disposal of excess quantities of flammable vapors. Such a flare system generally includes manual and automatic pressure relief equipment, collecting lines (flare collection laterals), a main flare header, and a flare stack. Typically, a number of.

refinery or petrochemical units (herein termed plants) may vent gases, through collecting lines individual to each plant, into a common main flare header (or manifold) which leads to a common flare stack. A pilot light is ordinarily maintained at the flare tip (upper end of the stack), to ensure ignition of the vapors.

A flare system such as described, including as it does a high stack open to the atmosphere at its upper end, can operate under a slight vacuum if there is present in the stack a mixture of gases lighter than air. (Considerable quantities of hydrogen, for example, may be included in the mixture of stack gases).

Operating a flare system under a slight vacuum poses the continuing threat that air can be drawn into the system through a leak (resulting from a cracked flare line, for example) or an inadvertently opened fitting. The drawing of air into the system is highly hazardous, because this can result in the creation of an explosive mixture which, in the presence of the constant flame (pilot light) at the flare tip, could cause an explosion in the flare stack, with consequent damage. If hydrogen is being handled in the flare system, the hazard is particu larly great, since hydrogen has a very wide explosive range; roughly 5 percent oxygen in hydrogen is flammable.

In flare systems such as described, oxygen analyzers have been considered necessities, as they offer continuous measurement regarding air in-leakage. Such analyzers, located at the base of the flare stacks, are monitored continuously. Readings between 0.3 and 1.0 volume percent are the signal to operators to start a search for the source of the air.

For purposes of doing maintenance work'on the flare system, such as removing an automatic safety (pressure relief) valve, putting a flange in the line, etc., it would be possible, of course, to produce a positive pressure in the system (thus causing gas to leak out, rather than air to leak in, when the system is opened) by introducing large quantities of steam or thousand pounds per hour, for example) at the flare tip, or by dumping large quantities of heavy gases (such as butanes or ethanes) into the flare system. These expedients, however, are quite expensive, so that to continuously maintain a positive pressure on the flare system in this manner would bequite uneconomic, and therefore impractical. It may be here noted that the establishment of a positive pressure in the flare system would be called for, in order to perform maintenance operations on a safety device, for example, in all cases where there are no block valves isolating the safety devices; block valves isolating safety devices are not often used because such are generally looked upon with disfavor.

Sometimes, water seals are used between the main flare header and the stack to prevent back flow of flame into the system in the event that the flow of flammable vapors is stopped. Such seals also operate (by setting up a back pressure) to maintain a positive pressure (2 to 4 inches of water) in the collecting lines (flare collection laterals). (As previously stated, it is desirable to maintain a small positive pressure on the flare system, rather than operating it under a vacuum, so that air will not flow into the system when a leak occurs or when-a system fitting is inadvertently opened.) However, when a liquid seal is used, slugging (a sudden rapid flow of vapor'to the stack) may cause the seal liquid to be blown out of the sealing device and out of the stack; to avoid this, a controlled supply of make-up water must be provided forv the seal device, and this is expensive. Also, adequate measures must be taken to prevent freezing of the water, both in the seal of the device and in the flare stack.

An object of this invention is to provide a novel method of and means for increasing the pressure in a line through which gas is flowing.

Another object is to provide a relatively simple and inexpensive arrangement for increasing the pressure in a flare line/system.

v A further object is to provide a novel method and apparatus for maintaining a positive pressure on a flare system.

A still further object is to provide, for a flare'system, an arrangement for increasing the pressure in the system, which arrangement is relatively insensitive to (relatively unaffected by) the quantity of flare gases flowing in the system.

The objects of this invention are accomplished, briefly, in the following manner: A stream of gas is injected at a high velocity into a flare line (e.g., one of the collecting lines of a flare system), in a direction counter to thedirection of flow of gases in the line. The gas injected may typically be nitrogen.

A detailed description of the invention follows, taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a simplified, partial schematic of a flare system utilizing the invention; and

FIG. 2 is a sectional view, on a enlarged arrangement according to the invention.

Refer first to FIG. 1. A main flare header 1 (which may be a pipe 30 inches-in diameter, for example, and may be thought of as a manifold) carries vapors or gases to be flared to a (vertical) flare stack 2 which is, for example, 30 inches in diameter and 300 feet high. The main flare header 1 and the stack 2 preferably form the downstream portions of an extensive and extended flare system. A number of refinery units (process units) or plants may-feed gases to be flared into the header 1, by means of collecting lines such as lines 3 and 4 illustrated. Collecting line 3 carries gases to be flared from a so-called ethylene plant to header 1. The gases to be flared may come from various process units in the ethylene plant, by way of manual or automatic pressure relief valves or equipment (safety valves), or from various sources of sweep gas.

Collecting line 4, which may for example be eight inches in diameter, carries gases to be flared from another plant, No. 2 Hydrogen Plant," to header 1,

scale, of an through a shutoff valve 5 which may be located closely adjacent to header 1. Again, the gases flowing through the collecting line 4 (and which are to be flared) come from various process units in the hydrogen plant, by way of manual or automatic pressure relief valves or equipment, or from various sources of sweep gas. There is an elbow 6 in the line 4 (to be referred to further hereinafter), located downstream of all other connections from the hydrogen plant to line 4.

As indicated by legend (at the upper end of the header 1 in FIG. 1), other process units (plants) are connected to the main flare headerl by way of collecting lines similar to lines 3 and 4.

According to this invention, a gas is injected into the flare line (collecting line) 4 at high velocity, in a direction counter to normal flow toward the main flare header 1, which results in increasing the pressure in the No. 2 Plant flare system, upstream of the point of injection. The injected gas is any suitable non-reactive gas free of uncombined oxygen, and may typically be nitrogen. This injected gas may be thought of as a counter flow nitrogen injection purge.

The purge or injection referred to may be provided by means of a short length of small-diameter (A inch in diameter, for example) pipe 7 (see also FIG. 2) which extends through the elbow 6 into a straight section of the 8 inch collecting pipe or line 4. The pipe 7 may be about 12 inches long, for example. The inner end of pipe 7 (in line 4) is open, and the outer end of this pipe (outside line 4) is connected through a throttle valve 8 to a pressured source of a suitable so-called purge gas. The purge gas used here should be a non-reactive gas (that is, a relatively inert gas) free of uncombined oxygen, and by way of example may be nitrogen, under a pressure of about 100 psi. As illustrated in FIG. 2, the velocity vector V of the injected nitrogen is opposite in direction to the velocity vector V of the flared gases normally flowing in line 4 toward the main header 1, so the nitrogen injection is a counter-flow type of injection or. purge. The nitrogen, supplied under high pressure to injection pipe 7, is allowed to expand to a much lower pressure (to wit, a pressure close to atmospheric) at the inner open end of this pipe, inside the flare line 4. This results in a very high velocity V, of the injected gas, approaching the speed of sound in nitrogen. The velocity V of the flared gases, although the flared gas flow varies widely in a flare system, is much lower than V and may typically be on the order of 50 mph.

The principle of operation of the counter flow injection or purge of this invention will now be explained. High velocity gas (velocity V,) is forced to turn around in the flare line 4 and flow back past itself to the main flare header 1 and stack 2, as indicated by the arrows 9. The kinetic energy of the injected gas, (MV,'/2, is converted to pressure (potential) energy as the V, disappears and then reappears again as a low kinetic energy [(MV, ]2] in the opposite direction. Thus, in effect, a pressure barrier or resistance to flow is inserted into the line 4, which barrier the flared gases must surmount before they can reach the main flare header 1. As a result, the pressure in the line 4 increases upstream of the point of nitrogen injection; since the injection point is downstream of all other connections to line 4, the pressure is increased throughout the entire system upstream of this injection point.

In a test of an arrangement constructed according to this invention, the pressure at the start of the test on the No.2 Hydrogen Plant flare system (consisting of an 8 inch manifold) was about 0.1 in. of water. This pressure was measured at a convenient point upstream of injection line 7, and was measured with respect to atmosphere. A 1% inch purge to the flare system inside the plant (this was not a counter-flow purge) was shut off, causingthe pressure to decrease to 0.2 in. of water.

Then, the counter-flow nitrogen purge (at line 7) was turnedon full; the pressure then increased to +6.0 in. of water. This shows that the counter-flow purge markedly increased the pressure in the flare system, and is readily capable of maintaining a positive pressure on the No. 2 Hydrogen Plant flare system. As stated previously, a positive pressure on the flare system is highly desirable, since this will prevent air from leaking into the system. 7

The higher the velocity V of the nitrogen which is flowing counter to the exhausting flare stream, the less nitrogen (term M of (MV )/2) would be needed to maintain a given positive flare system pressure. A

sr'naller diameter pipe (54 inch rather than 1% inch, for

example) could be used at 7 to increase the velocity V If a high enough velocity such as V could be developed, even the main 30 inch flare header 1 could perhaps be pressurized with avery small quantity of nitrogen, using a counter flow nitrogen injection. In any event, sections of the flare system of FIG. I (which is to say, other collecting lines such as line 3) can be pressurized (maintained at a positive pressure) individually, by counterflow nitrogen injection similar to that described in connection with line 4.

It is desired to be pointed out, at this juncture, that the counter flow direction of the nitrogen injection is important. It may be noted that, in the No. 2 Hydrogen Plant flare test previously described, a purge of nitrogen (located inside the plant area) similar in quantity to the counter flow purge, but injected at a right angle to the direction of flow of the flared gases, had little effect on the flare line pressure-to wit, a pressure increase of only 0.1 in. of water.

Although nitrogen has been mentioned hereinabove as a counter flow injection gas, other non-reactive gasesfree of uncombined oxygen could be used for this purpose. For example, carbon dioxide could be used, if available. In fact, this gas might be more advantageous than nitrogen, since its M, in the kinetic energy expression (MV, )/2, is greater than that of nitrogen (the density of carbon dioxide being greater than that of nitrogen).

An important feature of the flow-resistance or pressure-increasing arrangement of this invention (counterflow gas injection) is that it is relatively insensitive to (that is, is relatively unaffected by or unchanged by) the quantity of flared gases passing through the pipe 4, and thus is relatively unaffected by slugging or large dumpings of flared gases. This results because V is large compared to V,, and a velocity change is'what is utilized'in the invention. Even if the flared gas flow were to double (which would double V, to say 100 mph), the flow resistance would then be proportional only to the square of about 800, rather than to the square of about 750, as initially; this is a ratio of increase of only 1.1:1, approximately. On the other hand,

flow of the first-mentioned stream and at a counter velocity in excess of said finite velocity.

, 2. Method of increasing the pressure in a line through which a stream of gas is flowing at a finite velocity, which comprises injecting a stream of nitrogen into said line in a direction counter to the direction of flow of the first-mentioned stream and at a counter velocity in excess of said finite velocity.

3. Method of claim 2, wherein said counter velocity is high compared to said finite velocity.-

Patent No. 3,7 9 Dated April 10, 1973 Elbert N. Shawhan Inventofls) It. is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Assignee incorrect.

Sun Oil Chemical Company" should be ---Su nOlin Chemical Company-- Signed and" sealed this 2nd day of April 1971p (SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM P0-1050 (10- USCOMM-DC 60376-F'69 v. W U.$. GOVERNMENT PRINTING OFFICE ISIS 0-356-33 

1. Method of increasing the pressure in a line through which a stream of gas is flowing at a finite velocity, which comprises injecting a stream of a non-reactive gas essen-tially free of uncombined oxygen into said line in a direction counter to the direction of flow of the first-mentioned stream and at a counter velocity in excess of said finite velocity.
 2. Method of increasing the pressure in a line through which a stream of gas is flowing at a finite velocity, which comprises injecting a stream of nitrogen into said line in a direction counter to the direction of flow of the first-mentioned stream and at a counter velocity in excess of said finite velocity.
 3. Method of claim 2, wherein said counter velocity is high compared to said finite velocity. 